Abstract: The effect of additives on the structures and properties of various VPO catalysts used for the select tive oxidation of n-butane to maleic anhydride was investigated．A series of VPO catalysts were prepared by coprecipitation process with the addition of Zr‚Co‚Ce‚Zn‚Sn‚K‚Ba and Bi as promoters．The catalytic performance test was conducted in a fixed-bed micro-reactor at atmospheric pressure．The results show that the catalytic behavior is affected remarkably by additives．For n-butane oxidation activity‚the effect of various elements follows the order of VPO-Zr＞VPO-Co＞VPO-Zn＞VPO-Sn＞VPO-Bi＞VPO-Ce＞VPO-Ba＞VPO-K＞VPO．For MA （Maleic Anhydride） selectivity‚the promoting effect is in the order of VPO-Zr＞VPO-Ce＞VPO-Zn＞VPO-Sn＞VPO-Co＞VPO-Bi＞VPO＞VPO-Ba＞VPO-K．Test results of catalysts including XRD‚FTIR and TGA／DTG indicate that no remarkable change occurs in the bulk structure due to the addition of elements‚both for precursor and final catalysts．All precursor catalysts are composed of VOPO_4.0.5H₂O‚while in the final samples‚（VO）₂P₂O₇ is formed accompanied by less amounts of α-VOPO_4.XPS measurement suggests the enrichment of P‚V and O on the surface of catalysts‚and the amount of surface oxygen species is directly related to the catalyst activity．

Key words:  vanadyl pyrophosphate catalyst, additive, n-butane oxidation, maleic anhydride, surface oxygen species